COSMOS : Coherent Optical Sources using Micromolecular Ordered Structures
Lead Research Organisation:
University of Cambridge
Department Name: Engineering
Abstract
Liquid Crystal Phases are unique phases of matter that are fluid yet have a coil-like structure over a wide temperature range as the material changes from an optically isotropic liquid, where the properties are the same in all directions, into a structured phase where the molecules spontaneously align so that their average direction of alignment twists around the coil or helix axis. The ends of the molecules trace out a path that would look like a coiled spring and these materials are called cholesterics. Light travelling along the direction of the helix axis experiences a layer like optical structure with a repeat distance equal to a half turn of the helix. If this is of the order of the wavelength of light then this leads to bright colour reflections, rather like those seen with soap bubbles. This effect is used in liquid crystal forehead thermometers where the materials are chosen to reflect colour changes as the helix expands or contracts. In these materials one can also apply voltages to expand or contract the helix and hence the reflected colours. Since light has wavelike properties and each colour corresponds to a specific length of the wave (or more correctly wavelength), if we add lots of waves together we can generate giant waves and this process is known as amplification. This is essentially the process used in laser devices where mirrors are used to trap waves and amplify the intensity or brightness of the light. If one mirror is slightly imperfect then this bright light of one specific wavelength leaks out along the axis between the mirrors. We use a very intense flash lamp of many wavelengths to get the light into the system in the first place. Also between the mirrors we place what is known as a gain medium that absorbs light of short wavelength and converts it to a longer wavelength. This is essentially the process used in day-glow inks where ultra violet or blue light (ie short wavelengths) is absorbed and through a process known as fluorescence converted to green or red (ie longer wavelengths) emitted light. So in a laser we have light absorption and emission with the mirrors selecting the longer wavelength output light. This is why most lasers used as pointers during lectures are red. What we have done recently is use the cholesteric mirrors to trap and emit light and by including a flourescent dye we have been able to make molecular liquid crystal lasers. Since we can change the reflections by applying electric fields we can change the emitted colour in a very controlled way. You may have noted that liquid crystal displays are very thin, the layer of liquid crystal within the device is less than the diameter of a human hair (i.e. less than 10 microns). This opens the way, using cholesteric liquid crystals, including fluorescent dyes, to make microscopic lasers. We can also make polymer rubbery cholesteric films where we can use mechanical force to change the reflected or emitted laser light. In this proposal we will make new cholesteric and polymer liquid crystals incorporating fluorescent dyes to make highly efficient lasers or light emitting systems that can then be incorporated into microchip devices. We hope to be able to make materials stable over a temperature range of -20 to +80 degrees C where we can switch the output colour with low voltages (less than 10 V in times shorter than a few 1000s of a second). We will study how the efficiency of these devices depends on chemical structure, optical, electrical and mechanical properties, as well as viscosity and elasticity. We will then make microchip and fibre-optic devices as well as larger area 2-D flat laser films. We have discovered highly stable cholesteric materials that have, peculiarly, helices at 3 directions at right angles. We will try to incorporate these into 3-D lasers or light emitting cubes. Once we have mastered control of the properties of these lasers we will explore their uses in medical, sensor, and telecommunications devices.
Organisations
Publications
Song M
(2009)
Optically-Pumped Lasing in Hybrid Organic-Inorganic Light-Emitting Diodes
in Advanced Functional Materials
Choi S
(2009)
Electrically Tuneable Liquid Crystal Photonic Bandgaps
in Advanced Materials
Kabra D
(2008)
High Efficiency Composite Metal Oxide-Polymer Electroluminescent Devices: A Morphological and Material Based Investigation
in Advanced Materials
Kabra D
(2010)
Efficient single-layer polymer light-emitting diodes.
in Advanced materials (Deerfield Beach, Fla.)
Castles F
(2012)
Flexoelectric coefficient measurements in the nematic liquid crystal phase of 5CB
in AIP Advances
Castles F
(2011)
The limits of flexoelectricity in liquid crystals
in AIP Advances
Choi S
(2007)
Wavelength tuning the photonic band gap in chiral nematic liquid crystals using electrically commanded surfaces
in Applied Physics Letters
Schmidtke J
(2010)
Polydimethylsiloxane-enclosed liquid crystal lasers for lab-on-chip applications
in Applied Physics Letters
Mowatt C
(2010)
High slope efficiency liquid crystal lasers
in Applied Physics Letters
Huang Y
(2010)
Mechanical bistability in liquid crystal elastomer-wire composite actuators
in Journal of Applied Physics
Kerr C
(2011)
Effect of filament aspect ratio on the dielectric response of multiwalled carbon nanotube composites
in Journal of Applied Physics
Morris S
(2009)
Removing the discontinuous shifts in emission wavelength of a chiral nematic liquid crystal laser
in Journal of Applied Physics
Mowatt C
(2010)
Comparison of the performance of photonic band-edge liquid crystal lasers using different dyes as the gain medium
in Journal of Applied Physics
Fang J
(2011)
Conjugated zwitterionic polyelectrolyte as the charge injection layer for high-performance polymer light-emitting diodes.
in Journal of the American Chemical Society
Ji Y
(2009)
Polysiloxane surfactants for the dispersion of carbon nanotubes in nonpolar organic solvents.
in Langmuir : the ACS journal of surfaces and colloids
Coles H
(2011)
Ultrafast Switching Liquid Crystals for Electro-Optic Transmissive and Reflective Displays and Microscopic Lasers
in MRS Proceedings
Castles F
(2012)
Blue-phase templated fabrication of three-dimensional nanostructures for photonic applications.
in Nature materials
Castles F
(2014)
Stretchable liquid-crystal blue-phase gels.
in Nature materials
Morris S
(2013)
Liquid crystalline chromophores for photonic band-edge laser devices
in Optical Materials
Morris SM
(2008)
Polychromatic liquid crystal laser arrays towards display applications.
in Optics express
Chen Z
(2011)
Enhanced charge transport by incorporating additional thiophene units in the poly(fluorene-thienyl-benzothiadiazole) polymer
in Organic Electronics
Choi S
(2009)
The switching properties of chiral nematic liquid crystals using electrically commanded surfaces
in Soft Matter